Autotools, ext/gd: Ensure test program compiles without warnings

In case compiler is configured in some strict way it might emit warnings
or even errors in the future if these aren't casted to void.

Author: petk

ext/gd/config.m4

@@ -162,17 +162,22 @@ AC_CACHE_CHECK([for working gdImageCreateFrom$1 in libgd], [php_var],
 
 /* A custom gdErrorMethod */
 void exit1(int priority, const char *format, va_list args) {
+  (void)priority;
+  (void)format;
+  (void)args;
   _exit(1);
 }
 
 /* Override the default gd_error_method with one that
    actually causes the program to return an error. */
-int main(int argc, char** argv) {
+int main(void)
+{
   m4_if([$1],[Xpm],
   [char* f = "test.xpm"],
   [FILE* f = NULL]);
   gdSetErrorMethod(exit1);
   gdImagePtr p = gdImageCreateFrom$1(f);
+  (void)p;
   return 0;
 }])],
   [AS_VAR_SET([php_var], [yes])],